The present invention relates to an optical transmitting/receiving apparatus including a laser diode/photo diode (LD/PD) integrated module housed therein. Specifically, the present invention relates to an optical transmitting/receiving apparatus capable of suppressing electrical crosstalk and magnetic crosstalk.
Owing to a rapid widespread of a fiber to the home (FTTH) technology, it is becoming possible to easily stay connected to the Internet all the time at general homes. With the widespread of the fiber, an optical subscriber terminal apparatus (optical transmitting/receiving apparatus) is becoming downsized and low-priced (low-costed).
Up to now, an apparatus including a laser diode LD that transmits light (optical signal) and a photo diode PD that receives light as separate modules is in the mainstream, so the optical transmitting/receiving apparatus itself is large in size.
In view of the above, a small-sized LD/PD integrated module that terminates a single-core bidirectional optical fiber (optical fiber cable) is proposed (Patent Document 1). With regard to such a single-core bidirectional LD/PD module, the LD/PD integrated module is accommodated in a metal housing obtained by processing a metal plate into a box shape.
In the meantime, such an LD/PD integrated module has a structure in which a CAN package of an LD module and a CAN package of a PD module are electrically connected to the metal housing owing to an adhesive technique such as laser welding and have a common ground.
However, in a case where the LD module is driven at high frequency (LD modulation), owing to parasitic effects of stray capacitance (Cf) of lead pins and inner structural components of optical devices of the LD and the PD, stray inductance (Lf) of the metal housing portion, and the like, the ground (GND) of the LD/PD integrated module does not become ideal.
As a result, an electrical crosstalk path is formed between a transmission side (light emitting side) and a reception side (light receiving side) of the integrated module, and a magnetic noise (magnetic crosstalk) generates, with the result that reception sensibility deteriorates. FIG. 11 exemplifies a mechanism how such electrical crosstalk generates.
As a countermeasure against the above, it is considered to stabilize a ground potential by accommodating a circuit board mounted with the LD/PD integrated module in a metal case, fixing the metal housing of the LD/PD integrated module to the metal case, and allowing the metal housing electrically contact the metal case.
However, the following problems are pointed out. That is, the metal case makes the entire optical transmitting/receiving apparatus larger in size and is complicated in processing steps and heavyweight, and material cost of the metal plate increases.
Accordingly, there is proposed an optical transmitting/receiving apparatus in which an LD/PD module is accommodated in a plastic case which can be downsized, and is processed easily, lightweight, and low in cost.
However, in the case where an LD/PD module is accommodated in a plastic case, problems concerning electrical crosstalk and magnetic crosstalk come up again. In other words, the above-mentioned problems come up again as follows. That is, because there is employed a structure in which the LD/PD integrated module including an LD module accommodated in a metal housing and an PD module accommodated in a metal housing is fixed to the plastic case, ground of the LD/PD integrated module does not become ideal, an electrical crosstalk path is formed between the transmitting and receiving sides, and a magnetic noise (magnetic crosstalk) generates, with the result that reception sensibility deteriorates.
The following are related arts to the present invention.    [Patent document 1] Japanese Patent Laid-Open Publication No. 2000-180671    [Patent document 2] Japanese Patent Laid-Open Publication No. 06-138347    [Patent document 3] Japanese Patent Laid-Open Publication No. 2001-268020